I am trying to export my edited version of New Bark Town to a IMD file, but when I click on Nitro export I get a Bitmap error:
"Failed to open texture on material tree1 at path: untitled/tree1.png".
Please help me.

Thats true, keep those files of the edited maps, and try to edit different maps, some maps have png errors and some don't. I think its because only some were exported correctly in 16 colors, while some are exported and 253 colors.